Wars don't count in the definition of "spree killings," or "mass killings" in the sense that these are done by deranged individuals who are murdering just for the sake of murdering. I know some mass killings are politically motivated, or have some kind of "reason" behind them, but for the most part they're just individuals who want to murder as many people as possible before they kill themselves or get taken out by law enforcement.

I think modern history is anything after the Civil War. At least that's what I get out of the term. Who really knows. I think it's different for each individual.

I think it's just a dodge to put a qualifying adjective in so they can still say "worst" (which gets people's attention), even though it isn't the worst.

The first media reports I saw said "worst mass shooting in U.S. history" so I'm assuming they added the word "modern" in after people pointed out that it wasn't the worst mass shooting in U.S. history.